Precision electronic tuning device

ABSTRACT

A precision electronic tuning device for tuning a musical instrument, having an audio pre-amplifier, a frequency to voltage converter, a D.C. attenuator and a differential voltmeter, the differential voltmeter connected between the outputs of the D.C. attenuator and the frequency to voltage converter as a null indicator to indicate the voltage difference between the two. The audio pre-amplifier amplifies the signal from the musical instrument and its output is connected to the input of the frequency to voltage converter. The frequency to voltage converter changes the amplified frequency to voltage proportionally.

United States Patent 1191 Zeiser PRECISION ELECTRONIC TUNING DEVICE [76] Inventor: Donald K. Zeiser, 7425 Parkdale,

Clayton, Mo. 63105 [22] Filed: June 29, 1972 [21] Appl. No.: 267,648

[ Jan. 29, 1974 Primary Examiner-Richard B. Wilkinson Assistant Examiner-U. Weldon 5 7] ABSTRACT A precision electronic tuning device for tuning a musical instrument, having an audio pre-amplifier, a frequency to voltage converter, a DC. attenuator and a differential voltmeter, the differential voltmeter connected between the outputs of the DC. attenuator and the frequency to voltage converter as a null indicator to indicate the voltage difference between the two. The audio pre-amplifier amplifies the signal from the UNITED STATES PATENTS quency to voltage converter changes the amplified fre- Maxey Z quency to voltage proportionally 2,896,161 7/1959 Fox 324/79 R 0 3,472,116 10/1969 Schott 84/454 2 Claims, 4 Drawing Figures D C POWER & PRECISION SUPPLY G D C ATTENUATOR I C 21 l7/ l6"\ DC VOLTAGE II I3 I5 INPUT IO AUDIO PRECISION DIFFERENTIAL SIGNAL PRE-AMP FREQUENCY VOLTMETER TO VOLTAGE CONVERTER I2 I /I4 AUDIO D C VOLTAGE PATENTED JAN 2 91974 SHEET u 0F 4 82 C ii 84 4% +|2v DC 94 FIG. 4

BACKGROUND OF THE INVENTION This disclosure outlines a tuner for a guitar, although this principle can be used to tune any type musical instrument.

The classic method for tuning musical instruments is to use a reference tone which is sought to be matched by the person tuning the instrument. Thus the accuracy of tuning is dependent on the ability of a person to distinguish between two tones or frequencies, therefore requiring concentration, taking considerable time, and having an amount of error due to the individuals audio distinguishing ability Pitch pipes are used, but when used, still have to be compared to the musical instrument through human interpretation. Even electronic devices that emit tones have to be compared by ear, and have different tonal characteristics than the sound produced by a musical instrument, making these tones difficult of comparison.

The principal object of this precision electronic musical instrument tuner is to provide a device which enables a musical instrument to be tuned very accurately and quickly, with the elimination of human ear interpretation.

Another object of thisinvention is to provide a device that can be used by any amateur to tune musical instruments very accurately and quickly. To explain this object more fully, not only an amateur, but even a completely deaf person can tune an instrument just as,

or more quickly and accurately, than a professional musical instrument tuner.

Other objects and many of the attendant advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the drawings.

SUMMARY OF THE INVENTION In this invention, the musical instrument output is electrically coupled to an audio pre-amp lifier, which amplifies the signal and filters out some unneeded harmonics. The output of the audio pre-amplifier is connected to the input of a precision frequency to voltage converter, and this converter, which consists of an amplitude detector, shunt zener voltage regulator, temperature compensated monostable multivibrator, and an integrator, changes the varying incoming frequencies into precise and exact voltages proportionally. The output of this converter may be in the vicinity of millivolts per H The resistors in the precision D.C. attenuator are calculated to give an exact direct current voltage. The number of precision resistors used in the attenuator equals the number of notes of the musical instrument being tuned. In the case of a guitar, a six position switch selects which voltage from the attenuator is compared to the frequency to voltage converter, through the instrumentality of the differential voltmeter.

Therefore, an exact 82.3 c.p.s. (low E string of guitar) signal goes into the pre-amplifier, is amplified then changed into voltage, which would be 0.823 volts DC, and is then connected to one side of the differential voltmeter. One position of the switch is connected to the attenuators precision resistor whose output is 0.823 volts DC, and connected to the other side of said voltmeter. Thus the voltage to each side of the differential voltmeter is equal, and a null or zero reading is obtained, indicating the musical instrument is on tune. An increase or decrease of fall of the frequency of the musical instrument will show a rise or fall deflection on the meter. I

the time interval formula used for the frequency to voltage converter is, T: RC g [2VCC cs 1) VBE 2)/ cc VBE 2)] the threshold (Th) is the point where the transistor commences conduction; (Sat) represents the saturation voltage of the transistors.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram illustrating the operational circuitry in the tuning device;

FIG. 2 schematic drawing showing the audio preamplifier portion of the device;

FIG. 3 schematic drawing showing the precision frequency to voltage converter; and

FIG. 4 is a schematic drawing showing the precision D.C. attenuator and differential voltmeter components of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, input jack receives an audio signal from the musical instrument, which is the input of the pre-amplifier 11. The audio pre-amplifier, which has an automatic gain control circuit, and a low pass filter, amplifies the input signal, whose output is an amplified signal as at 12. The precision frequency to voltage converter changes this signal from frequency to voltage proportionally, and its output is 10 millivolts per Hz, at least in this preferred embodiment, although other value may be contemplated. The output of the frequency to voltage converter is a direct current voltage, as at 14, and is connected to one side of a voltmeter 15, more aptly defined as a zero center differential voltmeter. The other side of said voltmeter connects to attenuator 17, a DC. attenuator with an exact DC. voltage output representative of 10 millivolts per P1,. In other words, one output of attenuator 17 may be 0.823 volts, which would indicate a null on the meter if the pre-amplifiers input is 82.3 c.p.s.; or low E on a guitar musical scale. 1

Switch 18 has the same number of positions as notes of the musical instrument. (For-a guitar, a seven position switch may be used. Six positions representing the six strings and one position for a battery check.) Number 19 is the positive power supply terminal, and number 20 represents the negative power supply terminal. Diode 19 is a reverse polarity protection device, designed to protect the circuit if it should be connected improperly. Number 21 represents the 12 volt D.C. battery power supply, preferably eight size AA batteries, or can be operated from line using a transformer and a rectifier.

Input signal jack 10, as-also shown also in FIG. 2, accepts the musical instrument signal, with one side connected to positive 6.2 volts D.C. regulated, as along line B, and the other side to capacitor 23 which is in series connected to resistor 26, which is connected to input 34, the positive input of the operational amplifier 30.

positive 6.2 volts D.C. Capacitor 27 is connected in parallel with resistor 28. Terminal 31 indicates the positive power supply input and terminal 32 indicates the negative power supply input of the operational amplifier 30. Resistor 29 is connected in series between the positive input 34 and line B. Capacitor 35 is connected between the noninverting input 34 and collector of transistor 36, with cathode of capacitor 35 toward collector of transistor 36, whose emitter of said transistor is connected to line B. Resistor 39 is connected in series between the base of transistor 36 and line B, and resistor 40 is connected between the base of transistor 36 and the cathode side of diode 42. Anode side of diode 42 is connected to output 38, the operational amplifiers output. The positive side of capacitor 41 joins the cathode of diode 42, and the negative side of capacitor 41 joins line B. Transistor 36 is the automatic gain control transistor. Represented by circuit line 43 is the audio pre-amplifiers output, and this is 6.2 volts D.C., 2 volts peak to peak. Outlined portion 47 indicates an optional low pass filter consisting of a resistor 44, which is connected between operational amplifier output 38 and the frequency to voltage converters input, and capacitor 45 connected between resistor 44 and resistor 49. Switch 46 is optional and engages the filter when closed. Number 48 generally indicates that part .of the circuit that represents the audio pre-amplifier with automatic gain control of the circuit. Lines A and B are connection points between FIG. 2 and FIG. 3.

Referring to FIG. 3, sensitivity resistor 49 can be decreased in value to in'crease the sensitivity of the circuit. The emitter of transistor 50 joins to a positive 12 volts D.C., with 'its collector connects to resistorl. The other side of resistor 51 is connected between resistor 49 and the base of transistor 56 at connection point 52, which is maintained at a variable voltage depending upon the amplifier output. Resistor 53 connects between resistor 51 and ground. Resistor 54 is connected between 12 volts DC. and base of transistor 50, and resistor 55 is connected between base of transistor 50 and collector of transistor 56. The upper trip point of transistor 50 is 7.2 volts D.C., and the lower trip point is 5.2 volts D.C., the emitter of transistor 56 joins line B, the cathode of diodes 59 and 60, and the positive side of capacitor 61. The other sides of diode 60 and capacitor 61 are grounded. The anode of diode 59 connects to resistor 58 which is in series with 12 volts D.C. Point 63, which is between resistor 58 and anode of diode 59 is maintained at 6.8 volts D.C. regulated. The collector of transistor 56 joins capacitor 69, with the anode side of said capacitor being between resistor 98 and the cathode side of diode 70. The other side of resistor 98 connects to ground. The anode side of diode 70 joins the base of transistor 72, and resistor 67, and the other side of resistor 67 connected to circuit point 63, which is maintained at 6.8 volts D.C. regulated. The emitter of transistor 72 returns to ground. Circuit point 62 joins resistor 64, with said resistor being in series with the collector of transistor 66. The emitter of transistor 66 goes to ground, and the collector joins the anode of capacitor 65, with the cathode of said capacitor connecting to base of transistor 72. Resistor 68 is between the base of the transistor 66 and the collector of transistor 72, and resistor 71 is located between circuit point 62 and the collector of transistor 72. Resistor 73 connects between the collector of transistor 72 and the circuit point D. The positive side of capacitor 74 joins between the resistor 73 and point D, and the negative side of said capacitor is grounded. Circuit lines C and D of FIGS. 3 and 4 are connection points. The segment of the circuitry delineated by 75 represents an amplitude detector, segment 76 presents a shunt zener voltage regulator, segment 77 represents a temperature compensated monostable multivibrator, and portion 78 represents the integrator.

Referring to FIG. 4, meter 79 indicates a zero center differential voltmeter (preferably from plus 0.5 to neg ative 0.5 volts D.C.), which joins with switch 80 in series with the resistor 81, which connects to positive 12 volts DC. Circuit line C represents positive 6.2 volts D.C. regulated.

Potentiometer 82 is in series with resistor 83, which is series connected with resistor 85, which is also in series with resistor 87. This resistor likewise is in series with resistor 89, as are also the remaining resistors 91, 93, 95, and 96. The other side of resistor 96 is grounded. Resistor 97 has one side connected between resistor and resistor 96, and the other side coupled to circuit line C. Points 94, 92, 90, 88, 86, and 84 are connection points which connect to switch 19 of FIG. 1 and then to meter 79. The switch output is an exact DC. voltage.

Also, by way of illustration, the following parts list describes a series of components that may cooperate to provide operation of the preferred embodiment. Observing other variations, and the values of said components, can be changed to vary the operating character istics of the circuitry. Integrated circuit 30 741 operational amplifier Transistor 36 2N3643 Transistor 56 2N3643 Transistor 66 2N3643 Transistor 72 2N3643 Transistor 50 2N3638 Diode 42 1N270 Diode 60 1N753A Diode 59 1N4148 Diode 70 1N4148 Diode 19 1N4l48 Capacitor 23 (disc) 0.0.5 uF

Capacitor 27 (disc) 0.01 uF Capacitor 69 (disc) 0.001 uF Capacitor 41 (S.T.) 1.0 uF v Capacitor 65 (MMF) 0.22 uF Capacitor 45 (S.T.) 2.2 uF

Capacitor 35 (S.T.) 4.7 uF

Capacitor 24 (S.T.) 22.0 uF

Capacitor 61 (S.T.) 22.0 uF

Capacitor 74 (S.T.) 22.0 uF Resistors in ohms i 5% A Watt (Carbon) Resistor 25 100.0 ohm Resistor 26 10.0 K ohm Resistor 28 1,500 K ohm Resistor 29, 44 1.0 K ohm Resistor 39 1.0 M ohm Resistor 40 220.0 K ohm Resistor 49 10.0 K ohm Resistor 51 27.0 K ohm Resistor 53 150.0 K ohm Resistor 54, 55 5.6 K ohm Resistor 58 220.0 ohm Resistor 64 1.2 K ohm Resistor 68 12.0 K ohm Resistor 98 68.0 K ohm Resistor 81 180.0 K ohm Resistor 97 6.8 K ohm Resistor 96 15.0 ohm Potentiometer 82 500 ohm Resistors in ohms i 1% Watt (Metal film) Resistor 67 16.2 K ohm Resistor 83 332.0 ohm Resistor 85 169.0 ohm Resistor 87 102.0 ohm Resistor 89 97.6 ohm Resistor 91 73.2 ohm Resistor 95 133.0 ohm Resistor 93 54.9 ohm The operation of this invention is best described in two parts, the quiescent of bias voltages which exist in the instrument in the absence of an applied signal and the small signal conditions which exist during a typical tuning operation.

The quiescant voltages are as follows:

Amplifier 30 is a DC. coupled operational amplifier, being essentially a voltage actuated device, in that the input currents to the inverting and non-inverting inputs are essentially zero. Because operational amplifiers require symmetrical positive and negative power supply voltages with respect to the DC. voltage present at the inputs and outputs, and this invention operates from a single supply voltage, we have established a reference voltage of positive 6.2 volts D.C. (approximately the supply voltage) designated Line B. The establishment of this line as a signal reference permits the realization of symmetrical supply voltage necessary for proper operation of amplifier 30.

Line 37 is maintained at essentially the same DC. voltage as point 34 (+6.2VDC) by the amplifier 30 output. This occurs because line 37 constitutes the negative feedback path of amplifier 30, whose output tends to drive the differential input voltage (difference voltage between amplifier inputs) to a null. Since amplifier 30 is a voltage actuated device, no DC. current flows through feedback resistor 28, thus line 43 is also maintained at essentially +6.2 volts D.C. under quiescant conditions.

The output of amplifier 30 is D.C. coupled to the Automatic Gain Control input diode 42, and the Amplitude Detector input resistor 49. Since the Automatic Gain Control (AGC) circuitry consisting of resistors 39 and 40, capacitors 35 and 41, diode 42 and transistor 36 is referenced to line B, diode 42 and transistor 36 do not conduct during quiescant conditions.

Similarly, since the Amplitude Detector consisting of resistors 49,5 1,53,54,55 and transistors 50 and 56 has its input transistor 56 referenced to line B, neither tran-' sistors 50 or 56 conduct during quiescent conditions.

The small signal conditions are as follows:

The output of the musical instrument (essentially a decaying sinusoid) is connected to input 10, A.C. coupled by way of capacitor 23, attenuated by resistors 26 and 29, thus superimposing an A.C. signal on the quiescant DC. voltage present at non-inverting input 34 thus causing this input to vary slightly above and below its quiescent DC. voltage. This input produces an amplified in-phase output at 38, again superimposed on the quiescent DC. voltage whose amplitude will increase until a negative feedback signal at line 37 (inverting input 33) is obtained that essentially equals the instantaneous voltage (D.C. i A.C. component) present at input 34. Thus amplifier 30 output, point 38, varies above and below its quiescent DC. voltage (+6.2 volts DC). In the event the input signal is sufficiently large to cause output 38 to exceed approximately 3 volts peak-to-peak, diode 42 conducts when output 38 goes positive with respect to line B (half wave rectifying the output), charging filter capacitor 41 to a peak voltage of approximately one volt, which is sufficient to provide base drive to transistor 36, turning it on. The lowered output impedance of transistor 36 in series with capacitor 35, shunts resistor 29, thereby reducing the impedance between point 34 and line B, thus attenuating the AC. signal present at 34 and maintaining a relatively constant 3 volt peak-to-peak output at 38.

Transistor 56 conducts during the positive alternation present on line 43, when the instantaneous voltage with respect to line B exceeds approximately 1 volt, thus forward biasing this transistor emitter-to-base junction. A portion of transistor 56s collector current is used to turn on transistor 50 whose collector current increases the initial base current of transistor 56, thus providing positive feedback and rapid turn-on of 56. This rapid tum-on of transistor 56 essentially squaring up the input, thus providing a sharp negative transition present at its collector, which is differentiated by capacitor 69 to provide a negative trigger pulse at the cathode of diode 70. Transistor 56 is turned off during the negative alternation on line 43.

The Shunt Zener Voltage Regulator (76) has no effect on the small signal operation of the instrument (see quiescent voltage description).

The Temperature Compensated Monostable Multivibrator (79) is arranged such that transistor 72 is normally conducting while transistor 66 is normally turned off. Upon receipt of a negative trigger pulse at the cathode of diode 70, transistor 72 is turned off and transistor 66 is turned on for a time determined by the RC 1 time constant of capacitor 65 and resistor 69.

Integrator 78 is used to filter the positive pulses present at the collector of transistor 72 for comparison with the Precision D.C. Attenuator Outputs.

The operation of this invention, whose components have been reviewed, will be described, by way of example, or to how a guitar can be tuned. This invention first changes the frequency from the musical instrument into a voltage, and then compares this proportional voltage to a reference voltage. The electrical output of the guitar is connected to the conventional phase jack 10, with said jack connected to the input of the preamplifier 11. The invention is turned on by way of a switch, and the power supply energizes the preamplifier ll, the frequency to voltage converter 13, and the attenuator 17, with positive and negative 12 volts generated direct current. The zero.centermeter 15, will indicate full deflection to one side, depending on how the meters terminals are connected between the attenuator and the converter. One position of a six position switch 18, is selected, with this position connected between an attenuators precision resistor, and the meter. One guitar string is then strummed, (the one strummed is dependent on the switch position selected), and its signal is then amplified by the preamplifier. The arnplified signal from said pre-amplifier is then changed into a voltage in the frequency to voltage converter. This voltage is proportionally equal to the frequency of the string of the musical instrument,

and is connected to one side of the zero center meter. The individual then turns the tuning cog on the guitar until the zero center meters needle reads zero, or in tune. If the guitar string is flat, said needle will dip respectively left, and if sharp, it will move to the right.

The voltage from a resistor of the attenuator, being connected to the meter, by way of the six position switch, is'calculated through the selection of the value of said resistor to be equal to the voltage output of the frequency to voltage converter, when the string is in tune. Each string being individually tuned, is strummed, and the switch position is selected dependent on the chosen string. Each position of said switch furnishes a reference potential compared to each guitar string. I

For a guitar, six resistors are needed in the attenuator to make constant reference voltages that are compared with the six potentials from the converter, representative of the frequencies of the six guitar strings. For musical instruments with more strings or notes, more resistors are needed in the'attenuator, and more switches to furnish these potentials to the meter. For example, in tuning a piano, approximately 88 resistor-switch combinations may be used to initiate tuning after the tone is picked up by a microphone.

Other switches may also be needed to change the operating characteristics of said tuner for multiple frequency instruments.

Also, there may be another position on said six position switch, or a separate switch, that when engaged, indicates a reading on the meter. This would be the battery check switch, where thepower supply is from a DC. battery, and if the meter reads within a certain area, the battery is of sufficient potential to operate the tuner. If this reading is to low, or outside of the indicated area, the battery supply should be changed. Such a check switch would not be required where the supply power is a common rectified line current.

The above description is of the preferred embodiment and is generally illustrative of the invention.

Other variations within the principle of the invention.

may occur to those skilled in the art in light of this disclosure.

I claim:

1. An electronic tuning device fortuning musical instruments comprising a pre-amplifier, a frequency to voltage converter, a DC attenuator, a differential voltmeter, and a power supply, said power supply connecting to and charging said pre-amplifier and converter, with said pre-amplifier amplifying the input signal from the musical instrument, said amplifier connected to said (a) frequency to voltage converter in which the frequencies are changed to voltages proportionally, said differential voltmeter being connected between the DC. attenuator and the frequency to voltage converter as a null indicator to indicate voltage difference.

2. An electronic tuning device for tuning musical instruments or the like, comprising a (an) amplifier for receiving and amplifying an electrical signal representative of a musical note from the instrument, a converter electrically connected to said amplifier and provided for changing the amplified signal into a corresponding representative voltage, a differential voltmeter designed for receivingsaid voltage from the converter, and attenuator being preset to emit one or more potentials representative of electrically tuned musical notes, said voltmeter provided for comparing the converter voltage with the attenuator reference potential to determine their difference, and a powersupply furnishing (to furnish) electrical energy to said amplifier and converter (circuit components). I

3. The invention of claim 2 wherein said preamplifier is a transistorized audio amplifier.

4. The invention of claim 2 wherein said' preamplifier is an integrated circuit'audio pre-amplifier.

5. The invention of claim 2 wherein said preamplifier has at least one filter to filter out harmonics from the notes and frequencies of the musical instrument.

6. The invention of claim 2 wherein said preamplifier has feedback components to vary the gain of said amplifier.

7. The invention of claim 5 wherein said filter is a low pass filter.

8. The invention of claim 2 wherein said preamplifier has an automatic gain control circuit.

9. The invention of claim 2 wherein said attenuator is a device whose outputs are exact voltages.

10. The invention of claim 2 wherein said voltmeter is a zero center meter that measures voltage differ ences.

11. The invention of claim 10 wherein said voltmeter is calibrated or marked to indicate if the instrument being tuned is flat, on tune, or sharp and its respective magnitude.

12. The invention of claim 10 wherein said voltmeter is a device for indicating a voltage difference.

, ply comprises a direct current battery.

17.'The invention of claim 2 wherein said power supply uses a diode between said supply and tuning device circuits as a reverse polarity protection device to insure the safety of the circuit components if power supply is 

1. An electronic tuning device for tuning musical instruments comprising a pre-amplIfier, a frequency to voltage converter, a D.C. attenuator, a differential voltmeter, and a power supply, said power supply connecting to and charging said pre-amplifier and converter, with said pre-amplifier amplifying the input signal from the musical instrument, said amplifier connected to said (a) frequency to voltage converter in which the frequencies are changed to voltages proportionally, said differential voltmeter being connected between the D.C. attenuator and the frequency to voltage converter as a null indicator to indicate voltage difference.
 2. An electronic tuning device for tuning musical instruments or the like, comprising a (an) amplifier for receiving and amplifying an electrical signal representative of a musical note from the instrument, a converter electrically connected to said amplifier and provided for changing the amplified signal into a corresponding representative voltage, a differential voltmeter designed for receiving said voltage from the converter, and attenuator being preset to emit one or more potentials representative of electrically tuned musical notes, said voltmeter provided for comparing the converter voltage with the attenuator reference potential to determine their difference, and a power supply furnishing (to furnish) electrical energy to said amplifier and converter (circuit components).
 3. The invention of claim 2 wherein said pre-amplifier is a transistorized audio amplifier.
 4. The invention of claim 2 wherein said pre-amplifier is an integrated circuit audio pre-amplifier.
 5. The invention of claim 2 wherein said pre-amplifier has at least one filter to filter out harmonics from the notes and frequencies of the musical instrument.
 6. The invention of claim 2 wherein said pre-amplifier has feedback components to vary the gain of said amplifier.
 7. The invention of claim 5 wherein said filter is a low pass filter.
 8. The invention of claim 2 wherein said pre-amplifier has an automatic gain control circuit.
 9. The invention of claim 2 wherein said attenuator is a device whose outputs are exact voltages.
 10. The invention of claim 2 wherein said voltmeter is a zero center meter that measures voltage differences.
 11. The invention of claim 10 wherein said voltmeter is calibrated or marked to indicate if the instrument being tuned is flat, on tune, or sharp and its respective magnitude.
 12. The invention of claim 10 wherein said voltmeter is a device for indicating a voltage difference.
 13. The invention of claim 12 wherein said voltage difference is proportionally equal to the difference between the frequency of the note generated from the musical instrument and the frequency that note should be.
 14. The invention of claim 2 wherein said frequency to voltage converter is a transistor frequency to voltage converter that proportionally changes frequency to voltage.
 15. The invention of claim 2 wherein said frequency to voltage converter has an input resistor as a sensitivity control.
 16. The invention of claim 2 wherein said power supply comprises a direct current battery.
 17. The invention of claim 2 wherein said power supply uses a diode between said supply and tuning device circuits as a reverse polarity protection device to insure the safety of the circuit components if power supply is reversed.
 18. The invention of claim 2 wherein said power supply furnishes direct current to said components.
 19. The invention of claim 18 wherein said direct current comprises a rectified alternating current supply.
 20. The invention of claim 9 wherein said voltages from the attenuator go through a switch that selects the correct potential to be compared by the voltmeter.
 21. The invention of claim 20 wherein said switch has an extra position for a battery check to ensure that the power supply is at sufficient potential. 